1. Technical Field
The invention relates to an electrostatic image developer, a process cartridge, and an image forming apparatus.
2. Related Art
In electrophotography, an electrostatic latent image is formed on an image supporting member (photoreceptor) though a charging step and a light exposure step, then the electrostatic latent image is developed with a toner, and the developed image is transferred onto a transfer material and fixed on the transfer material by heating or the like to obtain an image. Developers used in electrophotography can be broadly classified as one-component developers using only a toner having a colorant dispersed in a binder resin and two-component developers consisting of the above toner and a carrier. In a two-component developer, the carrier has functions such as stirring, transportation and charging and has separate its functions from those of the developer, thus attaining features such as excellent controllability and, therefore, two-component developers are widely used at present.
In recent years, digitalization processing has come to be used as a method that achieves high image quality, and by this digitalization processing, high-speed processing of complicated images has been made feasible. In a process of forming an electrostatic latent image on an image supporting member, a laser beam is used as an exposure light source, and the development of exposure technology with a small laser beam has enabled achievement of finer electrostatic latent images. By such image processing technology, electrophotography is extending to light printing and the like. In recent electrophotographic devices, higher speed and downsizing are required. Particularly with respect to full-color image quality, high resolution and high quality of a similar level to high-quality printing and silver halide photography are required. Consequently, it is important to maintain the electrification of a toner in the developer in order to faithfully visualize a finer latent image. That is, further improvement of the charging maintenance of a carrier having a charging function is required.
Meanwhile, carrier resistance is also significant in terms of its influence on higher image quality. In recent digital equipment, the carrier has a smaller diameter and lower resistance in order to cope with higher image quality. By reducing the diameter of the carrier, a fine image can be reproduced, and stable charging can be imparted even to a small-diameter toner. By reducing the resistance, the reproducibility of a solid image is improved and, in particular, the carrier is suitable for full-color high-density images.
However, when the resistance of the carrier is reduced, there is the problem of easy carrier scattering.
Various research has been conducted with a view to reducing carrier scattering. For example, there has been a proposal to increase the resistance of a developer or carrier, but when a carrier of extremely high resistance is used, development is extremely susceptible to an edge effect whereby a toner is developed easily in a region toward the edge of a latent image forming a solid portion and, as a result, problem occur such as reduction in the density of a solid image. For the purpose of improving developability, a method of regulating the resistance of a magnetic brush to within a certain range has also been proposed, but a reduction in the resistance of a carrier occurs due to the abrasion and release of a resin layer of the carrier during long-term use and, as a result, the adhesion of the carrier to an image supporting member occurs. It can hardly be said, then, that a carrier of such high resistance has a sufficient performance in terms of reduction in the adhesion of the carrier to an image supporting member.
When used in an actual machine, the electrification of a toner tends to decrease, for example, in a high-temperature high-humidity environment, and thus fogging easily occurs at high toner concentration, whereas while the electrification of the toner is increased at low toner concentration, so that image density is stabilized without generating fogging, carrier scattering onto a photoreceptor easily occurs, which may result in image defects such as white spots or pollution in the background. In a low-temperature and low-humidity environment, the electrification of a toner tends to increase, and thus the charge of the toner is too high at low toner concentration thus destabilizing image density in some cases. Carrier scattering also easily occurs to cause image defects such as white spots or pollution in the background in some cases. At high toner concentration, on the other hand, there may easily occur image defects or the transfer of a carrier to a photoreceptor due to a charge injection from the tonier to the carrier.
In such cases, when a carrier having a flat resistive characteristic (that is, less change in resistance) with respect to changes in toner concentration is used, the charge of a toner from low to high toner concentrations is stabilized, and carrier scattering is suppressed, so the carrier is effective in respect of fogging, image density and carrier scattering. Even if the toner concentration is arbitrarily changed, the respective characteristics are stable and hardly influenced by the environment.